Ethylene is produced by passing a feedstock containing naphtha, ethane, and other distillates through a furnace comprised of a series of tubes. To achieve desired creep strength, mechanical requirements, and oxidation resistance, these tubes are made of higher alloys such as the wrought Alloy 800 series, and centrifugally or static cast alloys such as HK, HP and HH alloys. The feedstock enters the furnace at a temperature of about 1000.degree. F. where it is heated to about 1650.degree. F. During the process pyrolytic coke is produced. Some of the coke accumulates on the walls of the furnace tubes. Nickel and iron in the tubes reacts with the coke to form long whisker-like structures that extend from the walls of the tubes called catalytic coke. These strands tend to catch pyrolytic coke passing through the tubes to form a complex amorphous coke layer on the inner wall of the furnace tubes. This amorphous coke layer acts as an insulator increasing the temperature of the inner walls in order to deliver adequate heat to the process stream to crack the feedstock. Consequently, the furnace must be periodically cleaned to remove this layer of coke. This cleaning is often called decoking. At many locations the tubes must be cleaned every 30 days.
The art has attempted to control catalytic coking by the selection of high chromium and nickel alloys with significant silicon content or by applying a chromium or aluminum or ceramic coating to the inner walls of the furnace tube. However, higher chromium coatings introduces instability in the alloy structures. Aluminum coatings have found limited success on wrought alloys with process temperatures not exceeding 1650.degree.. At higher temperatures inter-diffusion and spalling can occur. Solid ceramic coatings suffer from cracking and spalling.
Coatings of two or more materials have also been proposed for metals used in high temperature process applications. In Japanese Patent 80029151 there is disclosed a method for applying a chromium-aluminum-silicon coating. This coating is produced by a chromium pack cementation process followed by an aluminum-silicon pack cementation process. The coated metal is said to be useful for jet engines, gas turbines and internal combustion engines. In U.S. Pat. No. 3,365,327 there is disclosed a method for vapor diffusion coating metallic articles with aluminum-chromium-silicon to provide elevated temperature corrosion resistance for gas turbine and oil refinery applications. The technique involves a slurry coating followed by high temperature firing. There is no teaching in any of these references that such coatings would be useful for ethylene furnace tubes.
Pack cementation is a well known technique for applying diffusion coatings such as U.S. Pat. No. 5,873,951 where it teaches the use of a two step process with an intermediate cleaning and post process polishing. This coating combination of chromium-silicon and aluminum-silicon has proven successful up to 1850.degree. F. At temperatures exceeding 1850.degree. F. metallic coating combinations are prone to phase changes and reduced resistance to carburization. Consequently, there is a need for an effective method of treating high alloy ethylene furnace tubes to reduce catalytic coking